The response to an impulse can be used to characterize systems which are Linear and Time-Invariant (LTI). Specifically, many types of acoustic measurements rely on emitting a short pulse to excite a physical system, and then recording of the ensuing reflections in order to characterize the system. One generic example is the measurement of various types of rooms (from small broadcasting studios to concert halls) where there is interest in measuring their acoustic impulse response. Another example is in the measurement of various tubular systems, where measured reflections give indications of faults in the tubes. These types of measurements are termed Acoustic Pulse Reflectometry (APR).
One of the limitations or pitfalls of APR is that often, the presence of background noise can considerably reduce the accuracy. Several variants or augmentations to standard APR methods can result in better Signal to Noise Ratio (SNR) such as: 1) repeating the process many times and averaging the results, which is time consuming; 2) using pulse compression methods, such as Maximum Length Sequences (MLS) and swept sine, which are equivalent to sending many pulses in a relatively short time interval, hence resulting in enhanced SNR.
MLS is a pulse compression method, which is used widely. An MLS signal is a form of pseudo-noise, for example—a sequence of +1 and −1 values, having a length of (2N−1), where N is an integer. To facilitate the MLS method, the response to this signal is recorded, and correlated with the original sequence. It has been shown extensively in the literature that the result of this correlation is very close to the impulse response of the system being measured. Using N=16, for example, should theoretically increase the SNR with a factor of 256 (˜48 dB). However, in the real world the increase of the SNR does not reach the theoretical value.